The objectives of this research are the systematic study and explanation of two radiobiological phenomena of particular importance in tumors. Experiments will be done to explore further the kinetics of reoxygenation in solid tumors. The EMT6 tumor system in BALB/c mice will be used most extensively, because of its capability for assay of clonogenic survival. Other mouse tumors will be sought and employed in analogous experiments in order to broaden the spectrum of material studied and thereby increase the validity of extrapolations to human tumors. Together with the kinetic studies, likely mechanisms responsible for reoxygenation will be explored, including microcirculatory changes as detected by radioisotope clearance, cell motility as affected by different drugs including warfarin, and shrinkage. The portion of this project concerned with reassortment is aimed at describing more fully this phenomenon as it pertains to the induction of cyclic changes in radiosensitivity by irradiation and possibly by chemotherapeutic agents. By investigating the growth kinetics of unirradiated and irradiated tumor and normal cells, means will be sought of controlling cyclic fluctuations in radiosensitivity and thereby increasing the therapeutic ratio of tumors. Related investigations will be concerned with the repair of sublethal and potentially lethal damage, as these interact with other events in bringing about reassortment and consequent changes in radiosensitivity.